When sawing lumber, the residual slabs, edgings and trimmings are usually chipped and delivered to a pulp mill for making paper pulp. Bark is normally not allowed in a pulping process for being difficult to bleach and therefore for showing as impurities on the finished paper products. For this important reason amongst many, logs entering a sawmill are generally stripped of their bark at an early stage in the lumber sawing operation.
One type of log debarking apparatus which is well known in the lumber industry is illustrated and described in U.S. Pat. No. 4,249,585 issued on Feb. 10, 1981 to Hayes R. Mellott. This rosser type machine has a pair of shafts on which is mounted a plurality of cleated wheels. A log is positioned on the wheels and is rotated while a carriage travels on a track along the machine. A rotating toothed roller mounted on this carriage scrapes the bark off the log.
One drawback with this type of machine is that when a lump or a large branch socket rotates on one of the wheels, the log is suddenly displaced from its normal axis of rotation. When this happens, several wheels adjacent to the hump are not in contact with the log surface, often causing a slipping of the wheels and an uneven rotation of the log. These sudden movements and uneven rotation of the log cause the debarking tool to jump, skip, or otherwise operate in an inconsistent manner, leaving bark at some places and gouging into the surface of the log at other locations.
It is a common trend in the lumber industry nowadays to strive at increasing the yield of every tree. The thickness of slabs is continuously reduced, and electronic optimizers are used to calculate the best combination of lumber pieces available from a given diameter log. Therefore a deep groove left by the debarking tool on the surface of a log is sometimes detrimental to an ideal recovery of lumber from that log.
Another common modem practice in the lumber industry is that quality inspections carried out at a lumber mill as well as by purchaser's associations are more and more stringent. Hence, a single deep groove left on the surface of a log by a debarking tool may sometimes degrade a piece of lumber taken from this log to a fraction of its normal value. Similarly, a load of wood chips containing bark in excess of a minimum standard is generally rejected entirely by a pulp mill or a penalty is imposed on the supplier.
For all these reasons, the debarking of logs is a very important part of a lumbering process. The recovery of lumber from a log, and the quality of wood chips obtained from slabs, edgings and trimmings depends greatly upon a good operation of the debarking apparatus at the infeed end of a sawmill. In this respect, the rosser type debarking apparatus of the prior art is restricted in its debarking performance by the aforesaid erratic rotation of logs.